The present invention relates to a method for controlling an automatic transmission unit comprising at least a transmission with different selectable gear ratios and a clutch and being associated with a propulsion unit for a vehicle.
The invention also relates to an arrangement for controlling an automatic transmission unit comprising at least a transmission with different selectable gear ratios and a clutch and being associated with a propulsion unit for a vehicle.
In the field of vehicles, for example heavy commercial vehicles, there is a constant requirement to minimize the fuel consumption. To a certain extent, this requirement can be achieved by means of today's sophisticated fuel control systems, in which injection of fuel to the vehicle's engine is determined based on a number of operating parameters such as the throttle position, coolant temperature, camshaft position and oxygen level in the exhaust system. These parameters are used for determining the necessary output power of the engine, which also controls the required amount of fuel to be injected into the engine.
In the case of a heavy transport vehicle which is equipped with an automatic transmission unit, it is known that said unit is configured for shifting by means of a transmission control unit which is arranged in order to select a suitable gear during driving. In this manner, torque will be transmitted from the vehicle's engine to the drive wheels.
During driving conditions where the vehicle is operating at zero throttle and while the vehicle is still in motion, the transmission can be configured to disengage the gears and allow the vehicle to enter a freewheeling mode, i.e. with the transmission being disconnected from the engine. During this freewheeling mode, the kinetic energy of the vehicle will cause the vehicle to move. If no engine brake is used, the fuel consumption can be minimized during such operating conditions.
A typical example of a situation when fuel consumption can be minimized is during driving on a downhill road section. During such a situation, the fuel injection to the engine can be cut off completely during a freewheeling mode while the vehicle is rolling and maintaining its speed without being assisted by the engine and without any application of the accelerator pedal by the driver. As mentioned, this is beneficial for the fuel consumption of the vehicle.
The term “freewheeling” (also known as “coasting”) is consequently used to describe operation of a vehicle with the transmission being disconnected from the engine. Generally, this may be achieved either by selecting a neutral gear in the transmission or by actuating a clutch.
Furthermore, cruise control operation is often used during driving with heavy transport vehicles having an automatic transmission. During such operation, a suitable cruising speed is set by the driver. If the vehicle should come to a downhill section of a road, the transmission may enter a freewheeling mode. Due to the fact that the vehicle is rolling downhill, its speed will increase gradually. If no other braking is applied during this situation, the vehicle will eventually reach a so-called braking speed, i.e. a speed at which the cruise control system must prevent the speed of the vehicle from increasing further by braking the vehicle. According to known technology, such braking can be provided by means of engine braking. Normally, this is obtained by re-engaging a gear in the transmission. In order to engage the gear, the speed of the input shaft to the transmission must be accelerated to be adapted to the new gear. This increased speed is normally carried out by injecting fuel into the engine. The amount of fuel depends on the difference in speed that needs to be overcome by acceleration of the input shaft.
Consequently, a problem with the above-mentioned approach is that fuel injection is actually carried out during engine braking, due to the process of increasing the speed of the engine while braking during downhill conditions. This is not beneficial for the vehicle's overall fuel consumption.
Consequently, although the method above constitutes a suitable way of braking the vehicle it also contributes to a relatively high fuel consumption during the engagement of the newly selected gear.
The patent document WO 2013/104617 teaches a method in a vehicle for reducing fuel consumption during freewheeling. More precisely, this document teaches a step of detecting a condition involving freewheeling and a step of controlling a slip device (for example a friction clutch or a torque converter) for coupling the engine and the transmission. In this manner, the engine speed can be maintained at a control speed by increasing the engine speed to match transmission speed according to the normal control parameters of the transmission.
Consequently, although there exist previously known solutions for decreasing fuel consumption, i.e. at a freewheeling condition during downslope, there is still a general need to further reduce the fuel consumption.
It is desirable to provide an improved method and arrangement for controlling an automatic transmission unit by means of which the fuel consumption of a vehicle can be minimized, in particular during driving on a downhill road section.
In accordance with an aspect of the invention, a method is provided for controlling an automatic transmission unit comprising at least a transmission with different selectable gear ratios and a clutch and being associated with a propulsion unit for a vehicle. The method comprises the following steps: operating said vehicle at a prevailing vehicle speed in a downslope with said automatic transmission unit in a neutral gear or disengaged from said propulsion unit; operating said propulsion unit in an idle mode of operation; controlling said automatic transmission unit so as to select a gear which is a relatively low gear with regard to said prevailing vehicle speed, while maintaining said automatic transmission unit in a neutral gear or disengaged from said propulsion unit; and engaging said clutch to be slipping between the propulsion unit and the automatic transmission unit in order to adapt the rotational speed of the propulsion unit to the gear ratio of said relatively low gear.
By means of the invention, certain advantages are obtained. Most importantly, the invention leads to an improvement of fuel consumption due to the fact that the engine speed does not have to be increased in connection with braking of the vehicle during downhill freewheeling conditions. Consequently, fuel consumption is minimized.
According to an aspect, the step of engaging said clutch is automatically activated when the prevailing vehicle speed of the vehicle is estimated to have reached a braking limit speed of said vehicle. Also, the clutch is suitably controlled for obtaining an optimal increase of speed of the propulsion unit in order to limit the heat energy developed in the clutch to a predetermined value.
According to another aspect of the invention, an arrangement is provided for controlling an automatic transmission unit comprising at least a transmission with different selectable gear ratios and a clutch and being associated with a propulsion unit for a vehicle. The arrangement comprises a control unit connected to said propulsion unit and said automatic transmission unit, said control unit being configured for operating said vehicle at a prevailing vehicle speed in a downslope with said automatic transmission unit in a neutral gear or disengaged from said propulsion unit; for operating said propulsion unit in an idle mode of operation; and for controlling said automatic transmission unit so as to select a gear which is a relatively low gear with regard to said prevailing vehicle speed, while maintaining said automatic transmission unit in a neutral gear or disengaged from said propulsion unit; wherein said control unit is also configured for engaging said clutch to be slipping between the propulsion unit and the automatic transmission unit in order to adapt the rotational speed of the propulsion unit to the gear ratio of said relatively low gear.
According to embodiments, the automatic transmission unit is a dual-clutch transmission unit or alternatively a single-clutch transmission unit.
The invention is particularly adapted for being used in heavy commercial vehicles but can be used in generally any vehicle with a controllable automatic transmission unit in which there is a need for minimizing fuel consumption.